In radar systems designed for measuring the velocity of (rate of movement toward or away from the radar) and distance to targets of interest, the best system in terms of performance is the fully coherent system. In a fully coherent system periodic pulses are transmitted through space, which pulses are in-phase relative to a reference standard, and therefore in-phase relative to one another. The fully coherent system exacts a penalty relative to other systems in terms of cost, size and weight. A second radar system, termed coherent-on-receive is one in which transmitted pulses vary in phase from one another. A sample of each transmitted pulse is utilized in signal processing equipment associated with the radar receiver to compensate for the phase difference from transmitted pulse to transmitted pulse.
Unfortunately, it has been found that the compensation is far from ideal such that considerable pulse to pulse error remains. It is common to use a doppler (high pass) filter to filter out low frequency signals related to stationary and very slow moving targets (termed clutter) leaving high frequency signals related to faster moving targets of interest. (It should be understood that, depending on the use to which the radar is put, certain slow moving targets may be the targets of interest and the fast moving signals may be considered clutter.)
The above-described pulse-to-pulse phase error which remains is largely of sufficiently high frequency to pass through the doppler filter. Furthermore, the high frequency components passing through the filter may be greater in amplitude than the signals representing the targets of interest.